For example, various information such as audio information or image information, etc, have been handled with ease by various communication terminal equipments with popularization of technology for realizing digitization of data and/or compression technology thereof, and preparation and expansion of communication system and/or service system for data, and availability thereof has been realized. Communication terminal equipments are small and light in weight and is excellent in portability, and can be used for a long time, and have no necessity of relay device, etc. so that connections to various communication systems can be realized. At the communication terminal equipment, at the transmitting/receiving unit, in order to carry out modulation/demodulation processing of analog high frequency signal, there is provided, e.g., high frequency transmitting/receiving circuit of the superheterodyne system or the direct conversion system, etc.
At the high frequency transmitting/receiving circuit, there are provided an antenna unit including an antenna and a changeover switch and serving to receive or transmit signal, a transmit/receive switching unit for carrying out switching between transmission and reception, a frequency converting circuit unit, a demodulation circuit unit, a modulation circuit unit, and a reference frequency generating circuit unit for supplying reference frequency, etc. At the high frequency transmitting/receiving circuit, there are provided various filters between respective stages, a Voltage Controlled Oscillator (VCO) for local oscillation in which capacity is caused to be variable, functional components such as Surface Acoustic Wave (SAW) filter, etc., a matching circuit, a bias circuit, and passive components such as inductor, resistor and/or capacitor, etc. In the high frequency transmitting/receiving circuit, for the above reason, the entirety becomes large and power consumption also becomes large. This is great obstacle to miniaturization and light weight, and realization of low power of communication terminal equipment.
With respect to the above-described voltage controlled oscillator, e.g., as described in the Japanese Patent Application Laid Open No. 82569/1997 publication, there is also employed a variable capacitor 100 in which the MEMS technology which forms very small electrodes or movable bodies, etc. on insulating substrate by the thin film technology or the thick film technology, etc. is used to thereby realize miniaturization. As shown in FIG. 1A, the variable capacitor 100 is composed of an insulating substrate 101, and a movable member 102 of which one end is cantilever-supported on one surface 101a of this insulating substrate 101.
At the insulating substrate 101, as shown in FIG. 1B, on one surface 101a, a rectangular drive electrode 103 and a rectangular detection electrode 104 are formed in the state where insulation therebetween is maintained with each other, and a pair of draw-out electrodes 105, 106 are formed in a manner positioned in correspondence with the supporting portion of the movable member 102. The movable member 102 has insulating property and elasticity, and is composed of a supporting portion 107 formed at one end portion, a fulcrum portion 108 formed on this supporting portion 107 in a projected manner, and a movable portion 109 integrally formed along one side portion of this fulcrum portion 108 and opposite to one surface 101a of the insulating substrate 101 with a predetermined spacing.
As shown in FIG. 1C, the movable portion 109 has an external shape sufficient to cover the drive electrode 103 and the detection electrode 104, and is adapted so that a first movable electrode 110 and a second movable electrode 111 are formed on the internal surface opposite to one surface 101a of the insulating substrate 101 respectively in correspondence with these drive electrodes 103 and 104. The first movable electrode 110 and the second movable electrode 111 are conducted from the internal surface of the movable portion 109 to the fulcrum portion 108 and the supporting portion 107, and the supporting portion 107 is respectively connected to the draw-out electrodes 105, 106 in the state fixed on one surface 101a of the insulating substrate 101.
In the variable capacitor 100 constituted as described above, when external bias voltage is applied to the drive electrode 103 and the draw-out electrode 105 connected to the first movable electrode 110, electrostatic force is generated between the drive electrode 103 and the movable electrode 110. In the variable capacitor 100, the movable portion 109 is attracted toward the drive electrode 103 side by this electrostatic force while allowing the fulcrum portion 108 to undergo elastic displacement. In the variable capacitor 100, opposite spacing between the detection electrode 104 and the second movable electrode 111 is prescribed in the state where electrostatic force and elastic force stored at the fulcrum portion 108 is balanced. Thus, taking-out of electrostatic capacity generated between these electrodes is carried out.
In the variable capacitor 100, by adjusting external bias voltage in a manner as described above, magnitude of electrostatic force is changed. Thus, opposite spacing between the detection electrode 104 and the second movable electrode 111 is also changed. Since electrostatic capacity generated between the detection electrode 104 and the second movable electrode 111 is proportional to inverse number of the opposite spacing, the variable capacitor 100 functions as capacitor of the variable-capacitance type.
Meanwhile, in the variable capacitor 100, as described above, external bias voltage is applied from the draw-out electrode 105 formed at the insulating substrate 101 to the first movable electrode 110 of the movable member 102 side. In the variable capacitor 100, parasitic inductance which serves as line resistance component is produced between the draw-out electrode 105 and the first movable electrode 110, and is connected in series with capacitor detected by the detection electrode 104 and the second movable electrode 111. Thus, LC resonator is constituted on the whole. Accordingly, in the variable capacitor 100, as the result of the fact that the parasitic inductance component becomes great, the entire resonance frequency is lowered. Thus, the frequency region where the variable capacitor 100 is operative as capacitor becomes narrow.
On the other hand, in the variable capacitor 100, in order to realize low power of equipment, it is necessary to employ a configuration such that the movable member 102 is driven by lower applied voltage, whereby large capacity change is produced between the detection electrode 104 and the second movable electrode 111. In the variable capacitor 100, as described above, external bias voltage sufficient to allow the fulcrum portion 108 to undergo elastic displacement is applied, whereby the opposite spacing between the detection electrode 104 and the second movable electrode 111 is changed. In the variable capacitor 100, consideration is also made such that, e.g., the fulcrum portion 108 is caused to be narrow beam portion to thereby reduce elastic displacement characteristic to realize low voltage drive. However, in the variable capacitor 100, by such countermeasure, there takes place the problem that wiring between the draw-out electrode 105 at the fulcrum portion 108 and the first movable electrode 110 becomes narrow so that line resistance component becomes large.